IMO, soapstone always cracks. To minimize the crack propagaton you can drill a hole, with a standard 1/8” bit, just ahead of the crack. They use to do this on aluminum aircraft. The idea is that a circular hole distributes the stress around the circle edge which otherwise would be concentrated on a specific spot causing the crack to continue. They also use(d) this trick on windshield cracks. Be sure you are slightly ahead of the crack, or it doesn't help.
Hopefully your stone is supported in the oven. If so, the crack(s) are less of a concern. On the other hand, if the soapstone is it's own structure - meaning it spans from side to side w/o support - a crack could be an issue. I'm guessing it wouldn't fail when heated, but when the stone cools and shrinks, it could fall and those buggers are heavy.